Conventional dilution assays such as bacterial, serological, neutralization and CPE involve determining an effect or lack of an effect in a plurality of wells of a multi-well assay plate. Typically the plate has 96 wells each of which must be inspected separately by a highly skilled individual. This procedure is very time consuming, tedious and expensive. More recently systems have been developed for automated reading of multi-well assay plates, e.g. U.S. Pat. Nos. 3,773,426 and 4,115,010. Such systems operated on the principal of average optical density of each individual well. By recording only an average reading, however, much useful information is lost.